C# Academy · Lesson

Custom Types Supporting Ranges

Add Index and Range support to your types.

Lesson 4 of 413 steps

Custom Types Supporting Ranges is a free C# Academy lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Two Ways to Support Indices and Ranges

Your own types can opt into Index and Range syntax. There are two recipes: provide explicit indexers, or provide a Length/Count property plus a Slice method.

Explicit Index Indexer

Add an indexer that takes System.Index and the ^ operator just works on your type.

using System;

class Deck {
    private int[] cards = { 1, 2, 3, 4, 5 };
    public int this[Index i] => cards[i];
}

var d = new Deck();
Console.WriteLine(d[^1]); // 5

Explicit Range Indexer

To support slicing with .., add an indexer that takes System.Range and returns the slice type you want.

using System;

class Deck {
    private int[] cards = { 1, 2, 3, 4, 5 };
    public int[] this[Range r] => cards[r];
}

var d = new Deck();
Console.WriteLine(string.Join(",", d[1..3])); // 2,3

The Length + Slice Pattern

The compiler can also synthesize range support automatically if your type has a Count or Length property and a Slice(int start, int length) method.

Implementing the Pattern

Here a custom buffer exposes Length and Slice. The compiler rewrites buf[1..3] into buf.Slice(1, 2) for you.

using System;

class Buffer {
    private int[] data = { 10, 20, 30, 40 };
    public int Length => data.Length;
    public int this[int i] => data[i];
    public int[] Slice(int start, int length) {
        var result = new int[length];
        Array.Copy(data, start, result, 0, length);
        return result;
    }
}

var buf = new Buffer();
Console.WriteLine(string.Join(",", buf[1..3])); // 20,30

Why Length Matters

The Length (or Count) property is what lets the compiler resolve from-end indices like ^1 on your type. Without it, ^ cannot be translated.

using System;

class Buffer {
    private int[] data = { 10, 20, 30, 40 };
    public int Length => data.Length;
    public int this[Index i] => data[i.GetOffset(Length)];
}

var buf = new Buffer();
Console.WriteLine(buf[^1]); // 40

Resolving Index Manually

Inside an Index indexer, call i.GetOffset(Length) to convert a possibly-from-end index into a plain offset.

using System;

class Ring {
    private string[] items = { "a", "b", "c" };
    public int Length => items.Length;
    public string this[Index i] {
        get {
            int offset = i.GetOffset(Length);
            return items[offset];
        }
    }
}

var r = new Ring();
Console.WriteLine(r[^2]); // b

Resolving Range Manually

For a Range indexer, r.GetOffsetAndLength(Length) gives you both the start and the count to copy.

using System;

class Ring {
    private int[] items = { 1, 2, 3, 4, 5 };
    public int Length => items.Length;
    public int[] this[Range r] {
        get {
            var (start, len) = r.GetOffsetAndLength(Length);
            var slice = new int[len];
            Array.Copy(items, start, slice, 0, len);
            return slice;
        }
    }
}

var r = new Ring();
Console.WriteLine(string.Join(",", r[^3..^1])); // 3,4

Count vs Length

The pattern accepts either name: arrays and spans use Length; List<T> uses Count. Either property satisfies the compiler.

using System;
using System.Collections.Generic;

var list = new List<int> { 1, 2, 3, 4 };
// List<T> has Count, so Index works:
Console.WriteLine(list[^1]); // 4

Designing Slice Return Types

Decide whether your Slice returns a copy (like arrays) or a view (like spans). Views are cheaper but share mutable state, so document the choice.

A Complete Custom Type

This Sequence supports both ^ and .. via explicit indexers and a Length property.

using System;

class Sequence {
    private int[] data;
    public Sequence(int[] d) => data = d;
    public int Length => data.Length;
    public int this[Index i] => data[i];
    public int[] this[Range r] => data[r];
}

var s = new Sequence(new[] { 5, 10, 15, 20 });
Console.WriteLine(s[^1]);                       // 20
Console.WriteLine(string.Join(",", s[1..^1]));  // 10,15

Quick Check

Confirm what enables range syntax on a custom type.

Answer it in the interactive lesson

Recap

You learned two ways to add Index/Range support to your types.

Explicit indexers taking Index and Range.
The implicit pattern: a Count/Length property plus Slice(int, int).
GetOffset(Length) and GetOffsetAndLength(Length) resolve from-end positions.

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Add Index and Range support to your types. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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